No serious injuries have been reported following the magnitude 5.9 quake

MEXICO CITY, April 21 (Reuters) - A magnitude 5.9 earthquake struck western Mexico on Sunday, shaking buildings in the capital, but there were no reports of significant damage or injuries, officials said.

The quake, initially reported as a magnitude 6.2 off the coast, was centered 33 km (20 miles) northwest of La Union, on the border between Guerrero and Michoacan states, close to the Pacific coast, the U.S. Geological Survey said.

State oil monopoly Pemex said shortly after the quake that the power supply to its Tula refinery was interrupted. Not long after, it reported electricity was coming back on line again.

Pemex said it was unclear if the tremor had caused the break in energy supply at the Tula refinery, Mexico's second biggest. It has a production capacity of 315,000 barrels per day.

A Pemex official could not say whether production had been affected, but added that any loss would be made up.

The quake was felt in Mexico City, about 325 km (200 miles) away from La Union. Buildings shook and residents briefly ran outdoors, but most returned inside shortly afterwards.

Several senior Mexican officials, including Interior Minister Miguel Angel Osorio Chong, said via Twitter there were no immediate reports of injuries or significant damage.

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